This library has been archived. Please use the replacement library: https://github.com/adafruit/Adafruit-MCP23017-Arduino-Library
To download, click "Download Source" in the top right corner. Then install as indicated in our tutorial: --> http://www.ladyada.net/library/arduino/libraries.html in a folder called Adafruit_MCP23008.
You will also need the Adafruit BusIO library: https://github.com/adafruit/Adafruit_BusIO
This is very much beta, it seems to work fine but its not optimized and doesn't currently suport the interrupt capability of the chip
Currently it can do: setting pin directions, inputs and outputs and turning on/off the pull-up resistors
To use: Connect pin #1 of the expander to Analog 5 (i2c clock) Connect pin #2 of the expander to Analog 4 (i2c data) Connect pins #3, 4 and 5 of the expander to ground (address selection) Connect pin #6 and 18 of the expander to 5V (power and reset disable) Connect pin #9 of the expander to ground (common ground) Pins 10 thru 17 are your input/output pins
Check the datasheet for more info: http://ww1.microchip.com/downloads/en/DeviceDoc/21919b.pdf
Enjoy and send pull requests!